1. Field of the Invention
The present invention relates to solid-state image-sensing devices, and particularly to a metal-oxide-semiconductor (MOS) or complementary-metal-oxide-semiconductor (CMOS) solid-state image-sensing device and a method for producing the device.
2. Description of the Related Art
As a type of solid-state image-sensing device, an MOS or CMOS solid-state image-sensing device is known that includes unit pixels each including a photodiode sensor and a switching device and that reads signal charge accumulated in the sensor by photoelectric conversion, converts the charge into a voltage or current, and outputs it. In the MOS or CMOS solid-state image-sensing device, MOS transistors or CMOS transistors are used as, for example, switching devices for pixel selection and switching devices for reading signal charge. Also in peripheral circuits such as a horizontal scanning circuit and a vertical scanning circuit, MOS transistors or CMOS transistors are used, so that there is an advantage in that the transistors can be produced together with the switching devices.
Hitherto, in a MOS or CMOS solid-state image-sensing device using pn-junction transistors as sensors, the sensors of pixels are formed so that the pixels are isolated in the form of an X-Y matrix by a device isolation layer resulting from local oxidation, i.e., a so-called xe2x80x9cLOCOS (local oxidation of silicon) layerxe2x80x9d.
As shown in FIG. 21, a photodiode 1 to be used as a sensor is formed by forming a p-type semiconductor well region 3 on, for example, an n-type silicon substrate 2, forming a device isolation layer (LOCOS layer) 4 resulting from local oxidation, and performing ion implantation of an n-type impurity 6 such as arsenic (As) or phosphorus (P) in the surface of the p-type semiconductor well region 3 through a thin insulating film (e.g., an SiO2 film) so that an n-type semiconductor layer 7 is formed.
In the sensor (photodiode) 1, it is necessary that a depletion layer be enlarged for increasing the photoelectric conversion efficiency so that even signal charge photoelectrically converted at a deeper position can be used.
In order to dope the n-type impurity 6 in the formation of the photodiode 1 to be used as a sensor, ion implantation is performed using a photoresist layer 8 aligned on the device isolation layer 4 to protect other regions, as shown in FIG. 21. Thus, a pn-junction j appears at an end A of the device isolation layer 4. It is known that a stress generates crystal defects such as dislocation at the end A of the device isolation layer 4. Accordingly, when the depletion layer, generated by reverse biasing the pn-junction j, occurs in the region of at the end of the device isolation layer, which has the crystal defects, a leakage current is increased by the electric field. When the leakage current is increased in the sensor (photodiode) 1, a signal charge is generated and forms a dark current, even if no light is incident. Since the dark current is generated by the crystal defects, each sensor 1 has a different amount of generated dark current, which appears as nonuniformity of the image quality.
Accordingly, it is an object of the present invention to provide a solid-state image-sensing device designed so that photoelectric conversion efficiency in sensor parts can be increased.
It is another object of the present invention to provide a solid-state image-sensing device designed so that a dark current resulting from leakage current can be reduced, and to provide a method for producing the device.
To these ends, according to an aspect of the present invention, the foregoing objects are achieved through provision of a solid-state image-sensing device having pn-junction sensor parts isolated corresponding to pixels by a device isolation layer. The solid-state image-sensing device includes a first-conductivity-type second semiconductor well region formed between a first-conductivity-type first semiconductor well region and the device isolation layer. In the device, when the device is operating, a depletion layer of each of the sensor parts spreads to the first semiconductor well region, which is beneath each of the sensor parts.
Preferably, the second semiconductor well region is simultaneously formed with the semiconductor well regions formed after the formation of the device isolation layer in a CMOS transistor.
According to another aspect of the present invention, the foregoing objects are achieved through provision of a solid-state image-sensing device having pn-junction sensor parts isolated corresponding to pixels by a device isolation layer resulting from local oxidation. The solid-state image-sensing device includes a semiconductor region of a conductivity type opposite to the conductivity type of a charge accumulating region of each of the sensor parts, and the semiconductor region is formed between the charge accumulating region of each sensor part and the device isolation layer.
Preferably, the solid-state image-sensing device further includes a second semiconductor well region formed between the device isolation layer and a first semiconductor well region beneath the device isolation layer, and when the device is operating, the depletion layer of each of the sensor parts spreads to the first semiconductor well region, which is beneath each of the sensor parts.
The semiconductor region may be formed by extending a portion of a second semiconductor well region formed between the device isolation layer and a first semiconductor well region beneath the device isolation layer.
According to a further aspect of the present invention, the foregoing objects are achieved through provision of a solid-state image-sensing device including pn-junction sensor parts isolated corresponding to pixels by a device isolation layer resulting from trench isolation. The solid-state image-sensing device includes a semiconductor region of a conductivity type opposite to the conductivity type of the charge accumulating region of each of the sensor parts, and the semiconductor region is formed to extend from the device isolation layer to a pixel region.
Preferably, the opposite-conductivity-type semiconductor region is formed by extending a portion of a semiconductor well region.
According to a still further aspect of the present invention, the foregoing objects are achieved through provision of a method for producing a solid-state image-sensing device which includes the step of forming, by performing ion implantation, a semiconductor region after forming a device isolation layer resulting from local oxidation, wherein the device isolation layer isolates pn-junction sensor parts in correspondence with pixels; the conductivity type of the semiconductor region is opposite to the conductivity type of a charge accumulating region of each of the sensor parts; and an end of the semiconductor region is positioned at the side of the parts except for an end of the device isolation layer.
Preferably, the semiconductor region is formed by a second semiconductor well region formed between a first semiconductor well region and the device isolation layer.
In the method, after forming the device isolation layer, the semiconductor region may be formed by forming, beneath the device isolation layer, a second semiconductor well region leading to a first semiconductor well region.
According to yet another aspect of the present invention, the foregoing objects are achieved through provision of a method for producing a solid-state image-sensing device which includes the steps of: forming a device isolation layer resulting from local oxidation, the device isolation-layer isolating pn-junction sensor parts corresponding to pixels, and for forming a gate electrode of a read transistor connected to each of the sensor parts; and forming, by performing ion implantation, a semiconductor region of a conductivity type opposite to the conductivity type of the charge accumulating region of each of the sensor parts so that an end of the semiconductor region is positioned at the side of the sensor parts except for an end of the device isolation layer, with the gate electrode being used as a reference position.
According to still another aspect of the present invention, the foregoing objects are achieved through provision of a method for producing a solid-state image-sensing device which includes the step of forming a semiconductor region of a conductivity type opposite to the conductivity type of a charge accumulating region in each of pn-junction sensor parts so as to surround a device isolation layer resulting from trench isolation, wherein the device isolation layer isolates the pn-junction sensor parts corresponding to pixels.
According to a more aspect of the present invention, the foregoing objects are achieved through provision of a method for producing a solid-state image-sensing device which includes the step of forming, after forming, on a semiconductor substrate, trenches for isolating pn-junction sensor parts corresponding to pixels, and after forming a semiconductor region of a conductivity type opposite to the conductivity type of a charge accumulating region of each of the sensor parts so as to surround each trench, a device isolation layer by embedding an insulating material in each trench.
According to the present invention, photoelectric conversion efficiency in sensor parts in a solid-state image-sensing device can be increased, which makes it possible to provide a solid-state image-sensing device with high sensitivity.
According to the present invention, sensor parts having high photoelectric conversion efficiency and a low dark current can be formed without increasing production steps.